/usr/share/X11/xorg.conf.d/10-monitor.conf

Section "Monitor"
        Identifier      "laptop panel"
        Option  "ignore"        "true"
EndSection
Section "Monitor"
        Identifier      "big display"
EndSection
Section "Device"
        Identifier      "onboard"
        Option  "Monitor-LVDS1" "laptop panel"
        Option  "Monitor-DVI1" "big display"
EndSection